CIRCULA TION. 433 



are frequency and regularity. Usually these can be appreciated not merely by 

 a physician but by any intelligent person. The physiological variations in 

 the frequency of the heart's beats have been referred to already (p. 412). In 

 an intermittent pulse the rhythm is usually regular, but, at longer or shorter 

 intervals, the ventricle omits a systole, and therefore, the pulse omits an up- 

 stroke. Either intermittence or irregularity of the cardiac beats may be 

 caused by transient disorder as well as by serious disease. 



Tension. When unusual force is Acquired in order to extinguish the pulse 

 by compressing the artery against the bone, the arterial wall, and hence the 

 pulse, is said to possess high teusionj or the pulse is called incompressible, or 

 hard. Conversely, the pulse is said to be of low tension, compressible, or soft, 

 when its obliteration is unusually easy. A very hard pulse is sometimes called 

 " wiry ; " a very soft one, " gaseous." High tension, hardness, incompressibil- 

 ity, obviously are directly indicative of a high blood-pressure in the artery ; 

 and the converse qualities of a low pressure. It follows from what has gone 

 before that the causes of changes in the arterial pressure, and hence in the 

 tension, may be found in changes either in the heart's action, or in the periph- 

 eral resistance, or, as is very common, in both. An instrument called the 

 sphygmomanometer 1 is sometimes applied to the skin over an artery, in order 

 to obtain a better measurement of its hardness or softness than the finger can 

 make. This instrument is not free from sources of error. 



Size. When the artery is unusually increased in calibre at each up-stroke 

 of the pulse, the pulse is said to be large. When, at the up-stroke, the calibre 

 changes but little, the pulse is said to be small. A very large pulse is some- 

 times called "bounding;" a very small one, " thready." Largeness of the 

 pulse must be distinguished carefully from largeness of the artery. The for- 

 mer phrase means that the fluctuating part of the arterial pressure is large 

 in proportion to the mean pressure. But if the mean pressure be great 

 while the fluctuating part of the pressure is relatively small, the artery, even 

 at the end of the down-stroke, will be of large calibre, while the pulse will 

 be small. 



It has been seen that the increased charge of blood which an artery receives 

 at the ventricular systole is accommodated partly by increased displacement of 

 blood toward the capillaries, and partly by that increase in the capacity of the 

 artery which is accompanied by the up-stroke of the pulse. The less the con- 

 tents of the artery the less is the arterial pressure, the less the tension of the 

 wall, and the more yielding is that wall. The more yielding the wall, the more 

 of the increased charge of blood does the artery accommodate by an increase of 

 capacity and the less by an increase of displacement. Therefore, a large pulse 

 often accompanies a low mean pressure in the arteries, and hence may appear 

 as a symptom after large losses of blood. In former days, when bloodletting 

 was practised as a remedial measure, imperfect knowledge of the mechanics 

 of the circulation sometimes caused life to be endangered ; for a "throbbing" 

 pulse in a patient who had been bled already was liable to be taken as an " in- 



1 From a(j>vy/i6f, pulse. 

 28 



